1. Field of Invention
This invention pertains to series-wound motors which may be operated across an AC source and more particularly pertains to series-wound universal motors. Furthermore, this invention pertains to motors of this type in which the speed of the motor must be regulated so as to cause the motor to run at a constant speed.
2. Description of the Prior Art
Many methods have been proposed to enable the speed of a series-wound electric motor to be varied and regulated. In prior methods, it has been proposed to utilize a resistance bridge in series with the armature winding of the motor. In methods such as these, the speed of the motor may be varied by altering the voltage across the bridge, causing motor speed to increase with increases in armature current, and causing motor speed decrease with decreases in armature content.
Such methods have two disadvantages. Firstly, such methods involve a loss of power in the resistance bridge. This power loss can be significant, especially at low motor speeds when comparatively large amounts of power are dissipated in the bridge. Therefore, the power curves of electric motors utilizing these methods are not as good as the power curves of unregulated series-wound motors. Secondly, it is entirely possible that as armature current is caused to increase, magnetic saturation will occur. Thus, the magnetic field within the motor becomes completely independent of armature current and the motor becomes non-regulatable. When saturation occurs, the regulating control system becomes unstable and precise control of motor speed becomes impossible.
It would thus be desirable to provide a series-wound electric motor, particularly a universal motor which can be used an AC source which is so designed that the speed of the motor could be accurately controlled, and which is so designed that instabilities arising from saturation in the motor upon increased armature current would be avoided. Moreover, it would be desirable to provide such a motor which would be power-efficient over its entire range of operating speeds.